Black contour artifacts when using OpenEXR renders with bloom and transparency background in VSE
At first I thought it has something to do with the blend mode of the material (settings panel: Opaque, Alpha Clip, Alpha Hashed, Alpha Blend). But, that seems not to be the matter.
The reason of this artifact lies probably within the AlphaOver blend mode in the VSE. You can find a very nice explanation with CGCoockie (Working with Alpha Edges for Compositing in Blender).
A possible work-around is the following:

Duplicate the strip with the 'blooming'shape.
Set the Alpha attribute in the Source panel to &quot;Straight&quot;.
Increase the scale of the duplicated strip a little untill the black artifact is covered.